1. Field of the Invention
This invention relates to a permanent magnet type synchronous motor having magnetic poles of a permanent magnet arranged on a rotor and having magnetic poles of a soft magnetic material which are alternately arranged with the magnetic poles of the permanent magnet, and particularly to the arrangement of the above two types of magnetic poles and their sizes.
2. Description of the Prior Art
Today, permanent magnet type synchronous motors, which have magnetic poles of a permanent magnet on a rotor and use a rotating magnetic field generated by a stator to rotate the rotor, are used in various fields. Such permanent magnet type synchronous motors are superior to an induction motor which has a winding on its rotor in enabling an increase in magnetic flux density per unit volume, thereby reducing the outside dimensions of the motors.
However, a hard magnetic material for use as a permanent magnet is generally expensive, and therefore, it is desired to use the hard magnetic material in a small quantity. In view of the circumstances, a rotor 1 having the cross section shown in FIG. 1 may be used. The rotor 1 has four permanent magnets on the outer wall of a rotor body 2 which is made of a hard magnetic material. These permanent magnets form hard magnetic poles 3. Between these adjacent hard magnetic poles 3, a magnetic pole 4 (hereinafter referred to as the soft magnetic pole) made of a soft magnetic material and being integral with the rotor body 2 is disposed. These soft magnetic poles 4 are made of the same soft magnetic material as used for the rotor. The hard magnetic poles 3 and the soft magnetic poles 4 have the same thickness radially, and are alternately arranged in the circumferential direction of the rotor 1. Hence, the rotor 1 has a cylindrical appearance.
Also, in a case of an ordinary 3-phase motor which has three slots per pole, arcuate lengths of the hard magnetic poles in the circumferential direction are about two times the slot pitch of a stator, and arcuate lengths of the soft magnetic pole are equal to the slot pitch.
A motor having the above rotor 1 generates torque with the hard magnetic poles 3 attracted by a rotating magnetic field generated by the stator and torque with the soft magnetic poles 4 attracted by the above rotating magnetic field. Therefore, this motor can generate greater torque compared with a motor using a rotor which disposes hard magnetic poles only on the outer wall of a cylindrical rotor without disposing the soft magnetic poles.
On the other hand, in the permanent magnet type synchronous motor, a back electromotive force is generated within the stator coil by rotation of permanent magnets. This phenomenon occurs in the motor having the soft magnetic poles 4 as shown in FIG. 1 and also in a motor without them. The back electromotive force increases as the motor rotates faster. When the back electromotive force becomes equal to the power voltage, the rotation cannot be further increased. Therefore, in order to obtain a faster rotation by means of the permanent magnet type synchronous motor, the stator is controlled to generate a magnetic field which counteracts the magnetic field of the above permanent magnet. This control is generally called a field-weakening control.
For the above permanent magnet type synchronous motor, its rotation torque may be increased by increasing the quantity of the permanent magnets to enhance a magnetic flux. However, it has a disadvantage that a motor cost is increased when quantity of the hard magnetic material is increased because the hard magnetic material for the permanent magnets is generally expensive as described above. Besides, there are disadvantages that the enhanced magnetic flux increases a voltage of the back electromotive force and lowers the upper limit of the rotating speed when the field-weakening control is not conducted. Also, when the quantity of the hard magnetic material is increased and the field-weakening control is required, the control range is increased and the power consumption is also increased. Besides, when a failure in a control circuit during the field-weakening control suspends the field control, there is a problem that the voltage of the back electromotive force exceeds a withstand voltage of the power circuit, causing a damage in the circuit.